1. Field of the Invention
The present invention concerns a tunable optical filter tuning device suitable for use in the receive part of an optical transmission system and adapted to reject part of optical noise superimposed on a wanted optical signal and having a broader spectrum than the latter.
2. Description of the Prior Art
To prevent such filtering deteriorating the wanted optical signal transmitted by an optical transmission system it is necessary to center the passband of the filter correctly relative to the transmission wavelength of the system, i.e. relative to the wavelength of the optical carrier modulated by the signal conveying information to be transmitted by the system.
A problem then arises in that this transmit wavelength is usually not known with great accuracy when the system is first put into service or when the system is returned to service after a fault in its optical transmitter or even in service due to drift of the wavelength of the optical transmitter.
To tune a tunable optical filter used in this way in an optical transmission system when the system is first put into service or returned to service a manual procedure is usually employed which controls the filter in such a way as to displace its characteristic along the wavelength axis whilst observing the output signal of a mean optical power detector at the filter output and to stop such displacement when a certain optical power is detected by the mean optical power detector.
A particular disadvantage of this procedure is the risk of tuning said filter to a wavelength for which the optical power detected by the mean optical power detector corresponds not to a wanted optical signal but to optical noise, and in particular optical noise with a high mean power produced by a receive optical preamplifier on the input side of said optical filter.
A further disadvantage of this type of procedure is that it is not automated and can therefore be a relatively lengthy process.
Document EP-A-0 534 644 discloses the automation of tuning of an optical filter for an optical transmission system in which a distinction is drawn between two tuning phases corresponding to two types of displacement:
a search phase in which the characteristic is displaced with a non-null mean slope until a pilot signal added to the transmitted data is detected; PA1 a control phase in which the displacement is controlled by a signal representative of the amplitude of the received pilot signal.
This tuning control device has the disadvantage of requiring additional circuits in the sending part to superimpose a pilot signal on the wanted signal. The receive part controlled in this way cannot be used with any conventional sending part without these additional circuits.
U.S. Pat. No. 4,164,650 described an optical filter tuning device including means for detecting a low-frequency modulation signal and deducing therefrom a correction signal to be applied to the filter and which tends to reduce to zero any error between the filter transmission wavelength and the transmission wavelength. The low-frequency modulation is produced locally by means of a squarewave signal generator and an adder which superimposes the correction signal and the squarewave signal. The correction signal is produced by a phase comparator comparing the phase of the squarewave signal and that of the low-frequency modulation signal at the output of the detector means.
This device does not include any means for automatic initial locking of the filter onto the transmission wavelength.
A particular drawback of this procedure is that said error cannot be reduced entirely to zero if it becomes too great relative to the correction capacity of said control system, determined by said amplifier and lowpass filter means.